Quantum Optomechanics with Superfluid Helium*

I will describe our work using single photon detectors to probe the motional states of a superfluid He-4 resonator of mass ∼ 1 ng [1]. The arrival times of Stokes and anti-Stokes photons (scattered by the resonator’s acoustic mode) are used to measure the resonator’s phonon coherences up to the fourth order, when it is initialized in a thermal state of mean phonon occupancy n_th∼1. The measurement backaction of such single photon detection is the heralded creation (or annihilation) of phonons in the mechanical mode. So, by postselecting on these detection events, we also measure the phonon coherences of the resonator when ≤ 3 phonons have been added or subtracted to the thermal state. Coherences and other statistics of k-quanta -subtracted or -added states of light are of interest in quantum metrology, quantum information, and quantum thermodynamics, and this work extends the potential use of such states to optomechanical platforms.

I will also describe ongoing efforts to realize Fock states and entangled states in the next generation of our optomechanical device (with a resonator mass ∼ 10 μg), and recent work to generate and probe coherent states of the mechanical resonator (displaced thermal states with <n> ∼ 40,000, <(Δn)²> ∼ 1). A sensitive measurement of their coherence allows us to bound the quantum gravity induced non-locality length scale to < 10^-18 m [2], comparable to the bound given by experiments at the Large Hadron Collider (< 10^-19 m).

[1] Y.S.S. Patil et al., Physical Review Letters 128, 183601 (2022)

[2] A. Belenchia et al., Physical Review Letters 116, 161303 (2016)